An objective of any forming method is to produce green parts which can be sintered to a shape that is reproducible to close dimensional tolerances and is free from defects. During green-forming and sintering, cracks, distortions and other defects can arise due to the shrinkage associated with the particle consolidation process. Thus, an object of a forming method is to produce homogeneous green bodies having adequate green strength.
Another objective of shape-forming methods is to produce articles having net shape, eliminating or minimizing the need for downstream operations, such as machining, to obtain final desired dimensions. Among the many shape-forming methods presently in use, dry pressing--which involves compaction of powder in a die--in particular requires machining and grinding to attain intricate shapes and close tolerances. Another method used to produce net shape parts, slip casting, while being capable of attaining close tolerances, is considered relatively slow for the manufacture of complex parts in high volume.
Injection molding, which is recognized as a premier forming method for complex ceramic shapes, overcomes the limitations of other forming methods. It is capable of rapidly producing close tolerance net shape, complex parts in high volume. In Fanelli et al, U.S. Pat. No. 4,734,237, and U.S. patent application Ser. No. 08/869,053, now abandoned the disclosures of both of which are incorporated herein by reference, processes for successfully molding net shape, complex parts in high volume are described.
Presently, net shape, complex ceramic parts are manufactured in injection molding equipment by formulating the powder feedstock material from new (unused) ceramic powders. For example, typical ceramic molding compounds are made from different combinations of Calcined Bone, China Clay (Kaolin) and China Stone (Cornish Stone) powders mixed in varying percentages, or different combinations of Bone Ash, Ball Clay, Feldspar, Kaolin and Silica powders mixed in varying percentages. Rejected fired ceramic parts are discarded and disposed of as landfill at considerable expense and time. The present invention obviates the need for such costly and time-consuming disposal of waste fired ceramic parts by providing useful compositions of reground ceramic powder mixed with other additives for making net shape, complex parts in conventional injection molding equipment.
The ceramic molding compositions according to the present invention provide readily moldable feedstock materials that obviate the need for high molding pressures and special debinding furnaces. The feedstock materials disclosed herein use water as the liquid carrier and can be molded at low machine pressures between about 150 to 1500 psi. The molded parts are dried before sintering by evaporation of the water, and thus the lengthy and complex debinding step typically required with polymer-based molding systems is eliminated.